Toner image fixing apparatuses are known, wherein a heater lamp is centered within a fixing or heating roll in order to evenly heat an inner surface of the roll. Typically, heat transfer from the lamp to the roll is inefficient because the lamp is spaced far away from the roll inner surface. As a result, a long warm-up time occurs once the lamp is energized. Another disadvantage associated with a fixing roll having a centered heater lamp is axial temperature droop. This problem occurs as a result of non-uniform heating along the roll inner surface caused by the boundary effect of lamp filament heat distribution and heat losses at the ends of the roll due to conduction of heat energy into journals, bearings, bushings and drive gears, as well as heat energy losses at the roll ends due to convection and radiation.
One attempt at solving axial temperature droop involves providing a heater lamp having a boosted filament, which produces more heat at the ends than in the center of the lamp. Thin steel or aluminum fixing roll cores do not transfer heat energy well in the axial direction; hence, the temperature of the core ends near the boosted ends of the lamp may be significantly higher than that of the core center portion near the non-boosted center portion of the lamp. If a thicker roll is used in combination with a boosted filament, then warm-up time is delayed, which is problematic.
Still a further prior art fixing roll implementation involves providing a roll core which is thicker at its center portion and thinner at its ends. This roll core results in a delayed warm-up time once a corresponding heater lamp is activated. In addition, this roll core requires extra processing during its manufacture resulting in higher costs.
Accordingly, a toner image fixing apparatus is desired wherein warm-up time is minimized and axial temperature droop is reduced.